When in “island” or “islanding” mode, a steam turbine (ST) power plant is electrically isolated from the outside grid to which the plant normally supplies electric power (hence the name islanding). Among other reasons, the plant might be placed into islanding mode because the electric power transmission system that it normally supplies has gone down and is cold. Or the plant might be placed into a standby state while the power transmission system is operational and powered from other sources, e.g., for plant maintenance. In such scenarios, the plant might be in islanding mode because of a planned occurrence or due to unplanned events. The length of the plant downtime might be known or not, and might be longer or shorter. It is advantageous to have an ability to maintain the operational status of elements of the plant, in a standby mode poised to return full load, efficiently and without risking undue stress or damage to the plant equipment.
In island mode while not connected to the grid, a power plant can be run at a very low output power level, for example producing only enough electric power to satisfy the “house load.” The plant is self-sufficient but not generating electric power to the grid. In addition to keeping essential plant functions operative, an advantage of running in island mode instead of shutting the plant down, is faster return to regular operation after unplanned events necessitating shutdown from full power operation, for example after a disruptive event that can be diagnosed and corrected in a reasonably short time but causes the breakers coupling the plant to the power grid to open.
The event might relate to conditions on the power grid or conditions in the plant. As a result, it may be advantageous to enter island mode when the power grid is down, in which case electric power for the house load is generated by the plant. In some scenarios, the plant may be shifted into a low power, idling or partially disabled mode while the power grid is functioning and electric power is available from the grid. In that case, electric power for the house load might be taken out of the power grid.
The house load refers to the electric power requisite to operate all plant auxiliary equipment (e.g., boiler feed pumps, circulating water pumps, building HVAC, lighting, etc.). In a nuclear power plant, the house load also includes the reactor coolant pumps and all other equipment requisite for the safe operation of the nuclear reactor, including for protection of the reactor when not operating normally. The actual level of the house load is dependent upon the specific plant design and, for example, might be between 5% and 15% of the rated ST load. Running a plant at such a low level, perhaps one twentieth of its rated capacity, can stress the operating equipment because elements are operated outside of their optimized ranges of temperature, pressure, flow rate, etc. Equipment issues can also result from other low load operational effects, such as erosion of last stage turbine blades due to spray cooling required at the turbine exhaust when operating at very low loads.
The island mode concept may be reserved for a “survival” mode typically entered in response to an unplanned “event.” A steam turbine power plant is not designed with the intention of running in island mode as a nominal mode of operation. Assuming that the event in question is the unexpected opening of the breaker that connects the power plant to the grid, within seconds the generator load drops from its present level (such as 100%, 80% or 90% of rated load) to about 10%. Feedback controls reduce the flow through steam turbine admission valves. Bypass valves may be opened to vent excess steam no longer being admitted to the ST (at a rate that can approach the full rated steam flow rate). Condenser pressure may increase substantially due to additional steam coming in, which requires design attention to the process of assuming island mode operation and the nature and duration of possible island mode operation. (Typically, above 5-6 inches of mercury, the condenser pressure is reaching alarm levels and may be approaching protective trip settings.)
For a plant equipped with islanding capability, an important goal is to enable smooth entry into islanding mode, resolving issues within the plant so operation resumes at the lower islanding rate, without damaging equipment or causing protective trips. Another goal is to operate at the low islanding rate safely and without undue stress on the equipment for a period of time. The need to enable transition into islanding operation may have been foreseen, and embodied in a plant design, but perhaps not to last for more than a few hours due to risks associated with low steam flow operation at low load, e.g., 10% of rated load or less. After that, the plant design might contemplate an orderly shutdown.
A primary risk associated with remaining in island mode for an extended time after the initial transient period is the risk of accelerated last stage bucket erosion due to the effects of hood sprays. One remedy is to reduce the condenser pressure by reducing the amount of steam generated in the boiler. In a nuclear power plant, this will be limited by the need to maintain at least the reactor's minimum stable load. Typically, reduced steam generation goes together with a reduction in main and reheat steam temperatures. This leads to “forced cooling” of the steam turbine high and medium/intermediate pressure (HP and IP) shell and rotor. Operation at lower than nominal design temperature leads to loss of radial clearance control and potential radial rub induced vibration. The severity of this effect is dependent on the magnitude of the forced cooling (the temperature difference below nominal), and the rate at which the cooling occurs (rate of temperature change per unit time).
Typically, grid events leading to islanding operation are expected to be resolved in less than two hours. After the initial stress of shifting into islanding mode, a short time in islanding mode does not present a undue risk of damage, for example to the steam turbine. However if the possibility of an extended grid event is high (such as several days) and it is desired to configure the plant to remain operational in islanding mode for an extended time, poised to shift back to regular operation without a costly shutdown, then methods and apparatus are needed for enabling the plant to operate in island mode with less risk, particularly without risk of damage to the main steam turbine. Advantageously, this need is met according to the invention, while likewise serving additional objects as will become apparent in this description.